In general, polyesters such as polyethylene terephthalates (PET) possess good mechanical, thermal, chemical and physical properties. Therefore, they have been widely used in preparing fibers, films, food and beverage containers, industrial mold products and others. Polyester films have recently been employed in laminating the interior of metallic food containers and the laminating process is carried out using adhesives, avoiding the conventional processes which use environmentally harmful organic solvents.
However, such laminating process is expensive mainly because the use of an adhesive is required. Also the adhesive may sometimes induce sanitation problems. Further, a laminated polyester film has poor post-processing properties, i.e., the polyester film is brittle and tears easily.
In order to impart better thermal adhesion and post-processing properties to a laminating polyester film, the use of various copolyester resins have been attempted.
However, copolyester resins prepared by conventional methods generally contain acetaldehyde at a level of more than 2.5 ppm, and the acetaldehyde content increases to an unacceptably high level of about 5 ppm when the copolyester resins are extruded and stretched to form biaxially oriented copolyester films.
Acetaldehyde, a material which is harmful to human, must not be present in a laminated copolyester film of a metallic food container at a level exceeding 3.5 ppm.